In the adjustable dockboard, as shown in U.S. Pat. No. 3,997,932, a ramp or deckplate is hinged at its rear edge to the loading dock and is adapted to be pivoted upwardly from a generally horizontal cross traffic position to an upwardly inclined position by a counterbalancing spring assembly.
A lip is hinged to the forward edge of the ramp and can be pivoted from a downwardly hanging pendant position to a partially extended position just short of its fully extended position by a lip lifting mechanism which is actuated as a consequence of the ramp being walked down from the inclined position toward the horizontal position.
The walkdown-type of lifting mechanism, as disclosed in U.S. Pat. No. 3,997,932, includes a lip lifting lug, pivoted to the ramp and having a surface adapted to engage the underside of the lip. The lug is connected through an elongated rod to one end of a link that is pivoted to the undersurface of the ramp. When the ramp is in the horizontal position, the link and rod are in an undercenter toggle position and as the ramp is elevated, a latching bar connected to the frame of the dockboard engages the link to thereby connect the link to the supporting structure. When the ramp is walked down by the operator toward the horizontal position, the relative movement of the ramp with respect to the latching bar causes the link to pivot and move the rod forwardly to thereby lift the lip toward its partially elevated position.
As described in U.S. Pat. No. 3,997,932, when the ramp has been walked down to a predetermined inclination, the link and rod move into an overcenter relationship and the latching bar is disengaged from the link. The overcenter relationship is maintained by the weight of the lip acting rearwardly through the rod.
When the ramp is walked down to a position where the lip engages the bed of a truck, the engagement of the lip with the bed of the truck will move the lip to the fully extended position, thereby releasing the weight of the lip from the rod and causing the toggle to break. With the toggle broken, the lip will automatically fall to its pendant position when the truck pulls away from the loading dock.
It has been found in the field, that a truck may inadvertantly back into the extended lip, and as the lip is rigidly held in the extended position, the impact can cause damage to the elongated rod, or other components of the lip lifting mechanism.
In addition, it has been found that when the truck includes end loads which are located on the end of the truck bed, the lip may not be able to be fully extended, but instead the lip may dig into the end load as the ramp is walked down. If a substantial load, such as a fork lift truck, is moved over the ramp in an attempt to fully extend the lip, it is possible that the lip lifting mechanism can be damaged.